The present invention refers to a cellular mobile communication system and a method in such a system for paging a mobile station.
An area covered by a cellular mobile communication system is subdivided into cells. In each cell, there is a radio base station, that serves mobile stations within the cell. Via a radio base station, communication can progress between the mobile station and a fixed network.
Further, the area of coverage is subdivided into a smaller number of location areas. Each location area comprises a plurality of cells.
The fixed part of the mobile communication system does not have any information on in which cell a mobile station in idle mode is positioned. On the other hand, it is registered in which location area the mobile station is.
When a network initiated link is to be set up to the mobile station, a Page is transmitted to the mobile station in all the cells within the registered location area. The mobile station receives the Page and responds in one of the cells. The continued communication is made in that cell. In a location area, there are generally a large number of mobile stations, and the number of Pages transmitted in the location area is in proportion to the number of mobile stations within the location area. A problem is that the capacity for transmitting Pages is not always corresponding to the need. For this reason, some Pages will be congested.
A solution to this problem is given in U.S. Pat. No. 5,590,398. The solution is that the mobile communication system, for each mobile station, collects statistics on in which cells the mobile station usually generates traffic and responds to Pages. By means of this statistics, a virtual location area is created for each mobile station. Pages to that mobile station are thereafter transmitted into the virtual location area. Since the virtual location area is smaller than the previously defined location area, the Page will be transmitted into a fewer number of cells, and thereby the total risk for Pages to be congested is diminished. A problem with the proposed solution is that it is hard to collect, update and arrange statistics for every participating mobile station in the mobile communication system. The solution is therefore difficult to implement. A further problem with the solution is that the statistics may lead to a trespass into personal integrity.
The present invention is directed to a problem in a mobile communication system, without means for collecting statistics for every mobile station, to diminish the risk of Pages being congested and therefore not being transmitted.
Another problem is that it may take a long time before the mobile station receives the Page. This problem is connected to the first one. Most often, the first problem may be reduced at the expense of the other, and vice versa.
A further problem is that a new method for transmitting Pages must not be difficult to introduce in existing mobile communication systems.
An object of the present invention is thus to limit the transmission of a Page to a smaller number of cells but retaining a large probability for the Paged mobile station to be reached by the Page.
The problem is solved according to the present invention by a cellular mobile communication system and a method in such a system, where the cells of the system are split up into two classes. Pages are primarily transmitted in the cells of the first class.
According to one aspect of the invention, a cell is assigned to the first class if it has a high capacity for transmitting Pages. According to another aspect of the invention, a cell is assigned to the first class if a large proportion of responses to Pages are received in the cell.
The invention is used, for example, in combination with grouping of cells into location areas. The mobile communication system registers in which location area each mobile station is present. When a Page is to be transmitted to a certain mobile station, it is first transmitted to cells of the first class within the location area which has been registered for the mobile station. If the mobile station does not respond within a predetermined time, the Page is then transmitted in more cells, e.g. all cells within the location area.
An advantage with the invention is that a fewer number of Pages need to be transmitted in order for the mobile station to be reached by one. In that way, less radio capacity is used for transmitting Pages. This radio capacity can instead be used for other radio traffic.
With the present invention, the probability is increased that a mobile station present in a cell with low capacity to transmit Pages will really be reached by a Page. In the cell, only a smaller proportion of the Pages are transmitted, i.e. such Pages which are intended for mobile stations which with a large probability are present in a cell with a low Page capacity. Since only a smaller proportion of the Pages will have to be transmitted in these cells, the risk of Pages being congested is diminished within them.
The invention has the advantage that essential gains in truncation can be made, since the limited number of channels dedicated for transmitting Pages is distributed so that a larger group of mobile stations listen to a larger common resource for transmitting of Pages. This reduces the risk of congestion. This solution and advantage is possible because the cell structure is nowadays to a large degree hierarchical. This means i.a. that cells have different maximal allowed highest signal strength. Mobile stations in idle mode primarily listen in cells with high signal strength. There is thus a larger chance that a mobile station will respond in a cell which is high in the hierarchical structure.
A further advantage is that it is simple to select those cells in which a Page is first to be transmitted. Existing mobile communication systems can therefore be simply modified in order to use the inventive method.
The invention will now be described in more detail by means of preferred embodiments and with reference to the attached drawing.